1979 300d heater

I just bought a new heater motor because my old one would blow the fuse contuinually. Sometimes instantly, or sometimes after 20 minutes. I took it to an electric motor shop and he pointed out how it was bad- the bearing were shot, which I wasn't suprised because it had been squealing awfully since I owned the car. However, after installing the new heater motor on monday, I went out today to find that the fuse was blown. What could be causing this? It's blowing the #8 fuse (16amp) and the White AC Crossover Idle stabilization one. Is the aux pump on this? I do have background in electrical, and I noticed that my aux pump runs with a little more than 1 ohm of resistance. Seems a little low.

Forums

Body and Interior Specific Aid

Chassis Model Number

W123

Start with motor

Recommend you start with motor and work backward to find the source of the high resistance or short. You will need to bench test the motor to see how many amps it is drawing. The electrical shop should be able to do that for you. If motor is ok then check amp draw again with motor in car. Work back through the speed resistors and switch panel to isolate the problem.

The motor is brand new from

The motor is brand new from you, ordered last friday. Should I still test it? Its $68 an hour for labor, one hour minumum so... after buying the motor Im a little drainged

Assume motor is ok

Since blowing the fuse was so common with your old motor its fairly safe to say the new motor is not the problem. It is just when troubleshooting an electrical problem you never want to assume anything. Without at a DC amp gauge that will read at least 30 amps it will be difficult to test the system. An electrical short detector might also be handy. Add to that the complexity of the system you might want to drive the whole car over to the electrical shop and have him isolate where the drain is coming from. He should be able to do it in less than an hour.

An electrical motor can be tested without an amp gauge if you are willing to rig up a little inline fuse holder. These can be purchased at any auto parts store. Put a 15 amp fuse in the holder and wire it directly to the motor you are testing. Use a battery or a 12 volt booster to power the motor directly. If it doesn't blow the fuse the motor is ok.

Visual check

One more thing you should check. Any evidence of contact burning at the fuse box holder. You should get under the dash and look at it from the back side. Previous motor could have burned the fuse contacts thus causing high resistance

Visual Check

Kent, Everything checks out good. Motor was tested at MT Electric Motors and he assured me it was fine. I really don't have the money to dig into the climate control, so, is the full on defrost unresisted? If so, I would think I could just wire up a fused direct 12 volt and run it off a seperate switch since its defaulted to hot anyway.

Running motor direct

You should be fine running the motor directly but you may get tired of hearing and feeling the high speed. Ask your electrical friend about getting some type of 12 volt (20 to 30 amp) adjustable reostat that you can wire inline to allow you to slow the motor down.